1. Field of the Invention
This invention relates to a phase change optical recording medium and a method for preparing the same.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical rewritable (or erasable) optical recording medium is of the phase change type wherein a laser beam is directed to the recording layer to change its crystallographic state whereupon a change of reflectance by the crystallographic change is detected. Optical recording media of the phase change type are of great interest since they can be overwritten by modulating the intensity of a single light beam and the optical system of the drive unit used for their operation is simple as compared with magneto-optical recording media.
Most optical recording media of the phase change type used Gexe2x80x94Te systems which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state. It was recently proposed to use new compounds known as chalcopyrites. Chalcopyrite compounds were investigated as compound semiconductor materials and have been applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib-IIIb-VIb2 or IIb-IVb-Vb2 as expressed in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by X-ray structural analysis and their basic characteristics are described, for example, in Physics, Vol. 8, No. 8 (1987), pp. 441 and Denki Kagaku (Electrochemistry), Vol. 56, No. 4 (1988), pp. 228.
Among the chalcopyrite compounds, AgInTe2 is known to be applicable as a recording material by diluting it with Sb or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s. See Japanese Patent Application Kokai (JP-A) No. 240590/1991, 99884/1991, 82593/1991, 73384/1991, and 151286/1992.
In addition to these phase change type optical-recording media using chalcopyrite compounds, JP-A 267192/1992, 232779/1992, and 166268/1994 disclose phase change type optical recording media wherein an AgSbTe2 phase forms when a recording layer crystallizes.
For prior art phase change type optical recording media, recording layers are formed using vacuum deposition equipment and remain amorphous immediately after formation. The recording layers must be crystallized by an operation generally known as initialization before the recording media can be utilized as rewritable media.
Initialization is carried out in various ways, for example, after a recording layer is formed on a substrate, by heating the substrate to the crystallization temperature of the recording layer for crystallization as disclosed in JP-A 3131/1990; irradiating a laser beam to the recording layer for crystallization, which method is called solid phase initialization, as disclosed in JP-A 366424/1992, 201734/1990 and 76027/1991; irradiating flash light to the substrate to achieve pseudo-crystallization by so-called photo-darkening, which method takes advantage of the photo characteristics of calcogen compounds, as disclosed in JP-A 281219/1992; and high-frequency induction heating the medium. JP-A 98847/1990 proposes to heat a substrate during formation of a recording layer to thereby crystallize the recording layer. JP-A 5246/1990 discloses a method involving the steps of forming a first dielectric layer, forming a recording layer thereon, heating it for crystallization, and forming a second dielectric layer thereon.
However, the initialization step by laser beam irradiation takes a long time and causes low productivity. Heating of the overall medium rejects the use of inexpensive resin substrates. That is, resin substrates can be distorted upon heating for initialization, causing tracking errors. The method of irradiating flash light is also low in productivity because several shots of irradiation are necessary to achieve full crystallization.
Under the circumstances, the use of a so-called bulk eraser is the only technique which is regarded commercially acceptable and currently used. The bulk eraser irradiates a beam from a high power gas or semiconductor laser through a relatively large aperture stop for crystallizing a multiplicity of tracks altogether. Since the bulk eraser permits the recording layer to be locally heated, the substrate temperature is elevated to a little extent, enabling the use of less heat resistant resins as substrates.
The bulk eraser, however, requires a time of several minutes for initializing optical recording discs of 12 cm in diameter. Then the initializing step is a rate-determining step in the making of optical recording discs. While TeGeSb base materials are currently most widely used for phase change recording layers, it is believed that the initializing operation cannot be removed insofar as these materials are used.
Prior art phase change type recording media require to repeat rewriting several times after initialization until a constant rate of erasure is reached. In most cases, rewriting is repeated about ten times before performance rating is carried out. The reason why the rate of erasure remains unstable upon rewriting immediately after initialization is that the formation of a AgSbTe2 or Inxe2x80x94Ge crystalline phase is incomplete.
To eliminate the initialization step which is required by prior art phase change type recording media, U.S. Ser. No. 08/598,913, entitled xe2x80x9cMethod for Preparing Phase Change Optical Recording Mediumxe2x80x9d and assigned to the same assignee as the present invention, proposes a method for forming a Inxe2x80x94Agxe2x80x94Texe2x80x94Sb base recording layer by separately effecting the step of sputtering Sb+In and the step of sputtering Ag+Te or by separately effecting the step of sputtering Sb, the step of sputtering In, and the step of sputtering Ag+Te. The recording layer formed by such a series of steps has been at least partially crystallized. After recording is repeated on the recording layer formed by this method so that the elements in the recording layer are fully diffused and mixed with each other, a sufficient change of reflectance is obtained as acquired after initialization by the bulk eraser. However, in the duration from immediately after the formation of the recording layer to several times of rewriting, the rate of erasure remains unstable like prior art phase change type recording media. More particularly, since reflectance is different between the region crystallized during formation and the region crystallized upon rewriting, the reflectance remains unstable until the rewritten regions are extended by increments throughout the entire surface of the recording layer. In the case of mark edge recording utilized in rewritable digital video discs (DVD-RAM), such reflectance variations can be mistaken for mark edges.
JP-A 106647/1996 discloses a phase change type recording medium comprising a recording layer in the form of AgInSbTe system artificial superlattice film having alternately deposited AgSbTe2 films and Inxe2x80x94Sb films or having alternately deposited AgSbTe2 films, In films, and Sb films. One of the alleged advantages is that the initialization energy required for the entire recording layer is reduced because the crystallized AgSbTe2 films are used.
We found that when an AgSbTe2 film and an Inxe2x80x94Sb film were stacked, as in the case of U.S. Ser. No. 08/598,913, the reflectance remained unstable in the duration from immediately after the formation of the recording layer to several times of rewriting. Also when an AgSbTe2 film, an Sb film, and an In film were alternately deposited, the reflectance remained unstable until rewriting was done several times. To acquire a stable reflectance in the crystalline region upon rewriting, the Inxe2x80x94Ge crystalline phase must be present in the crystalline region. However, in the embodiment of JP-A 106647/1996 wherein indium is not present in the AgSbTe2 film, but as the Inxe2x80x94Sb film or In film, it becomes difficult for indium to bond with tellurium to form an Inxe2x80x94Ge crystalline phase. Where initialization is carried out with low energy as described in JP-A 106647/1996, the Inxe2x80x94Ge crystalline phase cannot be fully formed during the initialization. For this reason, the reflectance remained unstable until the Inxe2x80x94Ge crystalline phase is fully formed by repeating rewriting several times. It is noted that specific initializing conditions such as linear velocity and laser power are described nowhere in JP-A 106647/1996.
In examples described in JP-A 106647/1996, the Sb films and Inxe2x80x94Sb films have a thickness of less than 5 nm. These films cannot be crystalline when their thickness is less than 5 nm. As a result, the reflectance of the recording layer immediately after its formation is very low. Since the low reflectance hinders focusing of a laser beam and hence, uniform heating, it becomes difficult to achieve uniform initialization.
Still further, the content of indium in the Inxe2x80x94Sb film is described nowhere in JP-A 106647/1996. In Example of JP-A 106647/1996, a laminate construction wherein indium and antimony are separated into In films and Sb films is simply compared with a single layer construction wherein indium and antimony are not separated, but formed into an Inxe2x80x94Sb film. It is thus believed that the composition of the Inxe2x80x94Sb film is the same as the combination of In and Sb films. Since the In and Sb films have the same gauge, it is believed that the indium content in the Inxe2x80x94Sb film is about 10 to 15 at %. Such a high indium content makes it difficult to form an Inxe2x80x94Sb film as a crystalline one even if its thickness is increased. There still arises the above-mentioned problem associated with initialization.
Therefore, an object of the present invention is to provide a novel and improved phase change optical recording medium wherein the manufacturing time is reduced and stable write/read characteristics are accomplished at the first overwriting as opposed to prior art optical recording media wherein the initializing step of the recording layer is a rate-determining step in their manufacture.
Another object of the present invention is to provide a novel and improved write-once type phase change optical recording medium which eliminates initializing operation and which cannot be rewritten at the same linear velocity as recording.
In a first aspect of the invention, there is provided an optical recording medium comprising on a transparent substrate a recording layer consisting essentially of at least one antimony base thin film and at least one reactive thin film. The antimony base thin film and the reactive thin film are disposed in close contact. The antimony base thin film is formed by depositing an Sb base material containing at least 95 at % of antimony to a thickness of at least 70 xc3x85. The reactive thin film is formed of a material which forms a phase change recording material when mixed with antimony.
Preferably, the antimony base thin film is crystalline.
In a first preferred embodiment of the optical recording medium, the reactive thin film is formed by depositing a Inxe2x80x94Agxe2x80x94Te base material containing indium, silver, and tellurium as major components or indium, silver, tellurium, and antimony as major components.
In the Inxe2x80x94Agxe2x80x94Te base material, the atomic ratio of indium, silver, tellurium, and antimony is typically represented by the formula:
(InxAgyTe1xe2x88x92xxe2x88x92y)1xe2x88x92zSbzxe2x80x83xe2x80x83(I-1) 
wherein letters x, y, and z are in the range: 0.1xe2x89xa6xxe2x89xa60.3, 0.1xe2x89xa6yxe2x89xa60.3, and 0xe2x89xa6zxe2x89xa60.5.
Preferably, at least one of the Sb base material and the Inxe2x80x94Agxe2x80x94Te base material contains an element M selected from the group consisting of H, Si, C, V, W, Ta, Zn, Ti, Ce, Tb, and Y, the content of element M in the recording layer is less than 5 at %, and the content of element M in the Sb base material is less than 5 at %.
In the recording layer, the silver may be partially replaced by gold; the antimony may be partially replaced by bismuth; the tellurium may be partially replaced by selenium; the indium may be partially replaced by aluminum or phosphorus or both.
In a second preferred embodiment of the optical recording medium, the reactive thin film is formed by depositing a Gexe2x80x94Te base material containing germanium and tellurium as major components or germanium, tellurium, and antimony as major components.
The number of interfaces between the antimony base thin film and the reactive thin film in the recording layer is preferably up to 20.
Preferably, when a reflectance is measured from the side of the transparent substrate, the recording layer as prepared has a reflectance Ro, and the recording layer having undergone repetitive recording includes a crystalline portion having a reflectance Rc and an amorphous portion having a minimum reflectance Ra, which are in the relationship: Ra less than Roxe2x89xa6Rc.
Preferably, the recording layer is irradiated with a laser beam at a linear velocity whereby the antimony base thin film-forming material and the reactive thin film-forming material are mixed to form a record mark, and the laser beam irradiation at the linear velocity is insufficient to crystallize the record mark. That is, when a laser beam is irradiated at the same linear velocity as used in forming a record mark, the record mark cannot be crystallized.
In a second aspect of the invention, there is provided a method for producing an optical recording medium as defined above, comprising the mixing step of continuously exposing the recording layer to a laser beam to thereby mix the antimony base thin film-forming material with the reactive thin film-forming material.
In the mixing step, the recording layer is irradiated with a laser beam at a linear velocity Vm which is preferably controlled relative to the linear velocity Vw at which the recording layer is irradiated with a laser beam during a rewriting step, so as to satisfy 0.2 Vwxe2x89xa6Vm. More preferably, the linear velocity Vm is controlled so as to satisfy Vwxe2x89xa6Vm.
Also provided is a method for producing an optical recording medium as defined above, comprising the steps of evacuating a sputtering chamber to lower than 0.5xc3x9710xe2x88x922 Pa, introducing a sputtering atmosphere gas into the chamber, and sputtering the Sb base material in the chamber to deposit the antimony base thin film.
Further provided is a method for producing an optical recording medium as defined above, comprising the steps of forming the recording layer, and heat treating the recording layer at a temperature of 60 to 120xc2x0 C.
In prior art phase change recording media, the single-ply amorphous recording layer formed by sputtering is initialized or crystallized by heating and slow cooling. When a rewriting or overwriting laser beam is irradiated after initialization, the recording layer is melted in the region where the recording power is applied and thereafter, quenched into an amorphous or microcrystalline state with a lower reflectance, forming a record mark. In the region where the erasing power is applied, no change occurs and the reflectance is maintained unchanged from that after initialization. Upon subsequent rewriting, the recording power is applied at sites where new record marks are to be formed and the erasing power is applied at the remaining sites. Whether the state before irradiation is crystalline or amorphous or microcrystalline, the sites where the recording power is applied are all converted into amorphous or microcrystalline record marks and the sites where the erasing power is applied all assume a crystalline state. Overwrite recording is enabled in this way.
In contrast, the optical recording medium of the present invention is produced by depositing an antimony base thin film and a reactive thin film and effecting a mixing treatment. The mixing treatment is to irradiate a laser beam to the recording layer to heat it in order to mix the elements of the antimony base thin films with the elements Of the reactive thin films. As a result of the mixing treatment, the recording layer assumes a state wherein amorphous phases such as Agxe2x80x94Sbxe2x80x94Te are dispersed in the Sb crystalline phase. Although the reflectance of the recording layer before the mixing treatment is relatively high due to the crystallized antimony base thin films, the mixing treatment reduces the reflectance. It is understood that the reflectance of the recording layer after the mixing treatment is still higher than the reflectance of amorphous areas or record marks.
The mixing treatment achieves the same effect as the initialization treatment used in prior art phase change recording media in the sense that the recording layer as formed is converted into a recordable state. Although the prior art initialization treatment crystallizes the recording layer to increase its reflectance, the mixing treatment according to the invention converts the recording layer into a state wherein amorphous phases are dispersed in the antimony crystalline phase, inviting a drop of reflectance.
After the mixing treatment, recording and rewriting (or overwriting) may be carried out in the same manner as in the prior art phase change recording media. More particularly, the recording layer is melted in the region where the recording power is applied and thereafter, quenched into an amorphous or microcrystalline state, forming a record mark. In the region where the erasing power is applied, crystallization of AgSbTe2 or the like occurs to increase the reflectance. Subsequent rewriting is carried out in the same manner as in the prior art phase change recording media.
According to the invention, the reflectances of the record mark and crystalline regions obtained upon first irradiation of rewriting laser beam after the mixing treatment are equal to those of the record mark and crystalline regions upon second or later irradiation of rewriting laser beam, respectively. That is, unlike the prior art phase change recording media wherein the single-ply amorphous recording layer is initialized, the optical recording medium of U.S. Ser. No. 08/598,913, and the optical recording medium of JP-A 106647/1996, the recording layer of the invention has a fully stable reflectance already at the first recording and rewriting.
In the optical recording medium of the invention, provided that the recording layer as prepared (that is, prior to the mixing treatment) has a reflectance Ro, and the recording layer having undergone repetitive recording includes a crystalline portion having a reflectance Rc and an amorphous portion (or record mark) having a minimum reflectance Ra, these reflectances are in the relationship:
Ra less than Roxe2x89xa6Rc. 
Note that the reflectance is measured from the side of the transparent substrate. The minimum reflectance of the amorphous portion is obtained when the reflectance becomes lowest as amorphous conversion proceeds to the most extent. The reflectance Ro of the recording layer immediately after its formation is generally lower than Rc, but relatively high due to the crystallized antimony thin film as previously mentioned, for example, at least about 60% of Rc. This enables precise control of the focusing of a laser beam irradiated for the mixing treatment while the mixing treatment becomes uniform. In the event where the reactive thin film is also crystallized, Ro can be equalized to Rc by optimizing the composition and thickness of both the thin films and optimizing the material and thickness of a dielectric layer and a reflective layer which are formed on the medium surface along with the recording layer. In this event, the mixing treatment can be removed.
According to the invention, the linear velocity at which the medium is rotated during the mixing treatment can be set significantly higher than the linear velocity at which the medium is rotated during conventional initialization treatment. This achieves an improvement in productivity.
In the conventional initialization treatment, the single-ply amorphous recording layer formed by sputtering is heated and slowly cooled for crystallization. In the case of phase change recording media, when the amorphous record mark is erased or crystallized by overwriting, the record mark is heated and then slowly cooled. Since the recording layer as formed and the record mark are common in that they are amorphous, but different in energy, the initialization requires higher energy and a lower linear velocity in order to lower the cooling rate. Herein, the linear velocity at which the rate of erasure during overwriting is less than xe2x88x9225 dB is designated a xe2x80x9crewritable linear velocity,xe2x80x9d and the linear velocity at which an optimum rate of erasure is available is designated a xe2x80x9crewritable optimum linear velocity.xe2x80x9d The linear velocity required for initialization is about ⅓ to about xc2xd of the rewritable optimum linear velocity. As a consequence, the initialization by irradiation of a laser beam requires a longer time.
In contrast, the present invention enables the relationship:
Vwxe2x89xa6Vm 
wherein Vm is the linear velocity at which the recording layer is rotated relative to a laser beam during the mixing treatment, and Vw is the rewritable optimum linear velocity after the mixing treatment. As a consequence, the time required for the mixing treatment is significantly shorter than the time required for the conventional initialization. The linear velocity Vm can be increased by increasing the power of a laser beam during the mixing treatment. Although the upper limit of Vm is not particularly specified, it is usually
Vmxe2x89xa65 Vw 
where an ordinary bulk eraser and recording device are used,
If the linear velocity Vm during the mixing treatment is made lower, the mixing treatment is enabled by a lower power laser beam. Then the necessary laser power can be very low when the mixing treatment is carried out at an equal linear velocity to the conventional initialization. However, in order to carry out the mixing treatment at a practical rate, the linear velocity Vm is preferably controlled to be:
0.2 Vwxe2x89xa6Vm. 
The optical recording medium of the present invention is well durably rewritable. A prior art Inxe2x80x94Agxe2x80x94Texe2x80x94Sb system medium fabricated by initializing a single-ply amorphous recording layer becomes substantially non-rewritable after about 10,000 cycles of repetitive rewriting because the rate of erasure drops. In contrast, the first embodiment of the present invention which is also a Inxe2x80x94Agxe2x80x94Texe2x80x94Sb system medium can maintain the rate of erasure over about 100,000 cycles of repetitive rewriting. Also the second embodiment of the present invention which is a Texe2x80x94Gexe2x80x94Sb system medium is more durably rewritable than prior art Texe2x80x94Gexe2x80x94Sb system media.
It is believed that such an improvement in rewriting durability is accomplished by stacking a reactive thin film and a crystallized antimony base thin film and subjecting them to mixing treatment as will be described later.
In the first embodiment of the invention, in order to provide an increased reflectance and an increased rate of erasure, a microcrystalline phase of Sb must be present in addition to the AgSbTe2 crystalline phase upon application of erasing power to the Inxe2x80x94Agxe2x80x94Texe2x80x94Sb system recording layer to effect crystallization. In the prior art method of initializing a single-ply amorphous recording layer, however, microcrystals of Sb are first formed upon initialization, silver is likely to diffuse into the antimony microcrystals, and the repetition of rewriting further promotes the silver diffusion, whereupon the function of antimony microcrystals is exacerbated. Accordingly, the rate of erasure lowers as rewriting is repeated. In contrast, according to the present invention, when a reactive thin film is mixed with a crystallized antimony base thin film, the silver (Ag) which has formed stable compounds of Agxe2x80x94Te and AgSbTe2 is mixed with the antimony (Sb) phase. Since silver is thus unlikely to diffuse into antimony microcrystals, the repetition of rewriting causes only a slight drop of the erasure rate.
Further, in the second embodiment of the invention, tellurium (Te) behaves as does silver (Ag) in the first embodiment. Since tellurium forms stable compounds of GeTe2 and Sb2Te3 before mixing with the antimony (Sb) phase, the diffusion of tellurium alone is suppressed, resulting in an improvement in rewriting durability.
The optical recording medium of the invention is described as the rewritable (or erasable) one although it may be used as a write-once type. In the write-once type application of the optical recording medium according to the invention, the treatment of mixing both the thin films should be removed. The write-once type optical recording medium according to the invention can be recorded, but cannot be erased when the above-mentioned rewriting method is used, that is, when a drive unit for rewritable recording media is used. More particularly, mixing of the antimony base thin film and the reactive thin film is possible when the recording power is applied, but crystallization is impossible in the region where mixing has occurred when the erasing power is applied to that region at the same linear velocity as in the recording mode. Since the optical recording medium according to the invention can have a relatively high reflectance immediately after preparation and achieve a substantial drop of reflectance as a result of mixing treatment, the invention accomplishes a write-once type optical recording medium eliminating a need for initialization and having excellent characteristics. The recording layer capable of write-once type recording can be established by properly selecting the composition and thickness of both the thin films.
While the first and second embodiments of the invention have the above-mentioned advantages, the first embodiment is superior in rewriting durability. Since the recording layer can be made thinner, the first embodiment has the additional advantages of reduced light absorption and a higher reflectance. This allows the degree of modulation to be markedly increased by the provision of a dielectric layer.